1. Field of the Invention
The present invention relates to a printer having a thermal head, and more particularly to a thermal transfer printer or a thermal printer with the improvement of a quality of a printed product obtained thereby.
2. Description of the Prior Art
In a thermal transfer printer, wherein a recording paper and a transfer film of an ink ribbon are laminated between a platen and a thermal head, the transfer film is heated as it is pressed against the recording paper by the thermal head, to melt or sublime a coloring agent on the transfer film and thereby transfer the resultant coloring agent to the recording paper. The thermal transfer printer is advantageous in that the thermal transfer printer does not require a spcial paper as the object recording paper.
However, such a thermal transfer printer is faulty in that the quality of a printed product obtained thereby varies depending upon the smoothness of the surface of the recording paper in use. Namely, when the recording paper is relatively rough, the coloring agent melted or sublimed by the thermal head is deposited on the high areas of the rough surface of the relatively rough recording paper and not spread into the low areas thereof. Consequently, the voids and the transfer failures occur on the printed dots or the transferred dots to cause the quality of the printed product to decrease.
FIG. 26 is an enlarged sectional view illustrating the contacting showing a thermal transfer recording operation and ondition of a thermal head 1, an ink ribbon 5 and a relatively rough recording paper 13 used as object recording paper. A heating resistor portion 4a of the thermal head 1, which is fomred on a partial glaze layer portion 3a consisting of glass, generates the heat at its portion when the electric current is applied thereto through an electrode 7a. This heat is transmitted to the ink ribbon 5 through a protective layer 8. As a result, he solid ink applied to a base film 10 of the ink ribbon 5 is melted, so that the transfer of an image starts.
When relatively rough recording paper 13 as an object recording paper is used, the difference between the top of a peak 13a of the relatively rough recording paper 13 and the bottom of a valley 13b of the relatively rough recording paper 13 is as great as about 10-28 .mu.m as shown in FIG. 26. Therefore, even if the ink layer 11 is melted with the ink ribbon 5 simply applied to such a surface of the relatively rough recording paper 13, the molten ink cannot enter the interior of the valley 13b of the relativley rough recording paper 12. so that the ink recording paper 13 which has the valley 13b. Accordingly, neither the motlen ink nor the sublimed ink are left in the low areas 13b of the relatively rough recording paper 13, they are necessarily left around the high areas 13a of the relatively rough recording paper 13 alone.
FIGS. 27 and 28 show printed dot 6 per one picture element. FIG. 27 shows an example of the printed image on the relatively rough recording paper 13, and FIG. 28 an example of the printed image on the smooth recording paper. It will be understood that the printed dots 6 on the relatively rough recording paper 13, which has the low transfer rate (T.sub.r), includes the large voids 6a and the large transfer failures 6b, and are not practical.
As stated above when the relatively rough recording paper 13 is used as the object recording paper in the conventional thermal transfer printer, the quality of the printed product markedly decreases. Namely, when the relatively rough recording paper 13 is used, the ink melted by the thermal head 1 is transferred only to the high areas 13a or the portion of surrounding around the top of the peak 13a on the surface of the relatively rough recording paper 13.
The conventional thermal transfer printer provides with a round platen, an ink ribbon cassette and a thermal head mounted on a carriage, and an ink ribbon is housed in the ink ribbon cassette. The thermal head of the conventional thermal transfer printer for use in improving the thermal transfer efficiency is disclosed in, for example, Japanese Patent Laid-open Publication No. 78868/1984 and Japanese Utility Model Laid-open Publication No. 41540/1984.
The thermal head consists of a head base, partial glaze layer portions or projections provided on the head base, and heating resistor portions provided on the tops of the partial glaze layer portions. However, the above thermal transfer printer using any of these thermal head is applied to the relatively rough recording paper, a printed product of a sufficiently high quality cannot be obtained, i.e., the above-mentioned faults cannot be eliminated.
The condition of the head contact in the conventional thermal transfer printer, will be described with reference to FIGS. 29 and 30. FIGS. 29 and 30 show a principal portion surrounding around the conventional thermal transfer printer, which is provided with a round platen 12 having a hardness (H.sub.s) of rubber of not more than H.sub.s 20.degree..
When a thermal head 1 in FIG. 29 is pressed with a force (F), the pressing force (F) is transmitted to the round platen 12 through the ink ribbon 5 and the relatively rough recording paper 13. If the hardness (H.sub.s) of the rubber of the round platen 12 is as low as H.sub.s 20.degree., the round platen 12 is indentated by (y) by the pressing force (F). Consequently, this pressing force (F) is also diffused to the head base 2 of the thermal head 1 to greatly decrease the unit area surface pressure (P), so that it becomes difficult to depress the high areas 13a or the peaks 13a of the surface of the relatively rough recording paper 13. Therefore, the printing on the relatively rough recording paper 13 is impossible.
Increasing the hardness (H.sub.s) of rubber of the round platen 12 may then be considered. If, however, the hardness (H.sub.s) of rubber of the round platen 12 is increased excessively, a flat surface portion of the round platen 12 cannot be obtained, so that the printing is still impossible.
In the conventional thermal transfer printer, the hardness (H.sub.s) of the round platen 12 is adapted not more than H.sub.s 20.degree. as stated above. When the hardness (H.sub.s) of the round platen 12 is is not more than H.sub.s 20.degree., the round platen 12 is greatly deformed by the pressing force (F) from the thermal head 1, and those portions of the thermal head 1 which are other than the partial glaze layer portions 4a and 4b of the thermal head 1 contact the round platen 12, as shown in FIG. 30, so that the unit area surface pressure (P) of the thermal head 1 decreases greatly. Consequently, the ink ribbon 5 does not closely contact with the relatively rough recording paper 13, and the voids and the transfer failures occur in the transferred dots or in the printed dots. This causes a decrease in the density of the printed image, so that the image is only partially reproduced.
According to the results of experiments made by the inventors of the present invention, the relation between the unit area surface pressure (P) from the thermal head and a transfer rate (T.sub.r) varies as shown in FIG. 1. Referring to FIG. 1, the lateral axis represents the unit area surface pressure (P), and the vertical axis the transfer rate (T.sub.r ), the curve a and the curve b indicating the characteristics of the smooth recording paper and the characteristics of the relatively rough recording paper, respectively.
The unit area surface pressure (P) in the conventional thermal transfer printer of this kind is around 0.1 kg/mm.sup.2, and the transfer rate (T.sub.r) obtained by this thermal transfer printer is about 70% when the smooth recording paper is used, and about 30% when the relatively rough recording paper is used.
The construction of a platen having a flat portion for a thermal transfer printer is disclosed in Japanese Utility Model Laid-open Publication No. 157642/1983. This platen is a platen having a combination of a flat portion and a curved portion. As shown in this laid-open publication, the characteristics of this platen reside in that the plate has at least one curved surface portion and a flat surface portion continuing tangentially from the curved surface portion.
Even on the platen which has the flat surface portion the platen shown in this laid-open publication, the contacting of a glaze layer portion of the thermal head occurs if the hardness (H.sub.s) of the rubber of the platen is low. With such a platen, the printing of the recording paper of the low smoothness cannot be done. Namely, merely providing the flat surface portion on the platen cannot ensure the high quality print.